Remote farms and crofts do have postcodes, though they may correspond to a local area rather than an individual property. Obviously they won't have house numbers. A farmer friend often has difficulty with this in trying to order things by telephone. You know the sort of thing...What's your postcode? Ok, now what's your house number? Haven't got one? You must have! No? Well, we can't process your order!
Brand new housing or business developments may be too new to have been given a street name or property number. It is a legal requirement for a local council decide these, and once they have, they will then inform Royal Mail, who will allocate a postcode. Their postcode allocation will normally take between 1 to 5 days.
Many people aren't aware that it's now a Royal Mail requirement that all parcels must bear the sender's postcode.